Acetic anhydride is a clear, colorless, liquid with a sharp, vinegar-like odor. Acetic anhydride is a commercially valuable chemical and can be found in a wide variety of industrial applications. Some primary applications of acetic anhydride include its use in the manufacture of cellulose acetate for films, plastic goods and coatings. Other applications include use in the manufacture of perfumes, explosives, synthetic fibers, sweeteners, polymers (e.g., polyoxytetramethylene glycol and polyacetal), weed killers, fungicides, various industrial chemicals, acetylsalicylic acid (aspirin), acetylcholine hydrochloride, acetophenacetin, sulfonamides, aceto-p-aminophenol, cortisone, acetanilide, theophylline, sulfa drugs, certain vitamins and hormones, and many other various pharmaceuticals and pharmaceutical intermediates not listed here. Acetic anhydride can also be used in the chemical treatment of papers and textiles and to produce acetyl ricinoleates, triacetin, acetyl tributyl citrate, and other plasticizers. Because acetic anhydride reacts with water, it is also sometimes used as a dehydrating agent in reaction mixtures where the removal of water is necessary.
It is known that acetic anhydride can be prepared via the reaction of acetic acid and ketene. Ketene, a valuable intermediate for the synthesis of many organic compounds, may be prepared by the thermal decomposition (continuous cracking) of acetic acid in the presence of a catalyst, e.g., triethyl phosphate. This reaction may be conducted at temperatures at or above 600° C. and under pressures ranging from about 10 to 50 kPa. Ammonia may be added to the mixture of hot cracked gas shortly after it leaves the reaction zone to neutralize the catalyst. Ketene is then isolated from the cracked gas and reacted with excess acetic acid to obtain crude acetic anhydride.
Typically, the ketene/acetic acid reaction is carried out in a system which includes an absorber and a scrubber. The majority of the ketene is reacted in the absorber stage. A crude acetic anhydride product stream exits the bottom of the absorber. Residual ketene, e.g., unreacted ketene, exits the absorber stage (via overhead gas stream) and is typically directed to a scrubber to convert the unreacted ketene into additional acetic anhydride. Although this process has been widely adopted throughout industry, the process requires the use of both an absorber and a scrubber to achieve the desired overall ketene conversion. These components, unfortunately, represent significant capital expenditures. Also, the continuous operation of these components leads to increased energy requirements for the process as a whole.
Thus, notwithstanding the above, there remains a need for processes for preparing acetic anhydride which both ease the burden of capital expenditures associated with existing processes, and offer high ketene efficiency and product yield. The present invention addresses these and other needs by providing processes for preparing acetic anhydride that utilizes a liquid ring vacuum compressor to react residual ketene exiting the reactor at significantly lower costs and increased efficiency.